The present invention relates to pulse transformers and more particularly to pulse transformers for providing triggering signals to semiconductor controlled rectifying devices while isolating controlling circuitry from the high voltages being switched by the semiconductor controlled rectifying devices.
Semiconductor controlled rectifying devices (SCR's), both unidirectional and bidirectional, are presently used for power control in many consumer and commercial devices. For example light dimmers and motor speed controllers employ such devices. Such devices are also often used in place of mechanical relays for switching power off and on in other equipment such as microwave ovens where precise turn-on time is important and control of operating duty cycle is desired.
A problem encountered in many consumer devices which employ SCR's is the isolation of the power line voltages from the control circuitry. The SCR's are triggered into a conductive state by applying a small voltage or current between a trigger input and one of the two main power connections of the device. As a result the controlling device, unless isolated in some manner, must float at the power line voltage. In some situations, such as light dimmers, floating of the very simple control circuitry is acceptable since by proper packaging the control knob or switch itself can be isolated. In other applications where the control circuitry is more complicated it is very desirable, and in some cases required by building codes or other laws, that the low voltage control circuitry be isolated from the power line voltages.
Trigger transformers have been used for such isolation purposes. The commonly available trigger transformers have been either of the bar type or the toroid core type. In the slug type, a small cylindrical bar of magnetic material, such as iron or ferrite, has been used as a core about which primary and secondary windings have been quickly and easily wound. In such an arrangement, there is not a continuous magnetic path so that the power capacity of the device is lowered. In addition, the isolation between primary and secondary windings has been generally limited to the insulation on the transformer windings themselves. The toroid core devices on the other hand, provide a very efficient magnetic path but are extremely expensive to wind. The insulation between primary and secondary windings on the toroid core is also normally limited to that carried by the windings themselves.
Thus it can be seen that it would be desirable to provide a pulse trigger transformer having the high efficiency advantages of the toroid core transformer, the ease of winding of the slug type transformer, and, in addition, a very high voltage isolation between the primary and secondary windings.